The present invention relates to a multi-layer plastic fuel tank, and more particularly to a multi-layer plastic fuel tank comprising at least one high-density polyethylene layer, at least one polyamide layer and at least one modified high-density polyethylene layer and well moldable at a low temperature. It also relates to a method of producing a multi-layer plastic fuel tank by forming its high-density polyethylene layer by reusing flashes, defective moldings, etc. of multi-layer plastic fuel tanks as regenerated products.
Polyolefin resins are inexpensive and strong, and excellent in a weathering resistance, a chemical resistance, etc., so that they are widely used for various containers, etc. However, the polyolefin resins do not necessarily have sufficient gas barrier properties. Accordingly, they cannot be used for fuel tanks for gasoline, etc. in countries having strict regulations.
In view of the above, various attempts have been made to provide thermoplastic resin compositions having excellent mechanical strength and gas barrier properties by blending polyamides such as nylons having excellent gas barrier properties with the polyolefin resins (Japanese Patent Laid-Open Nos. 54-123158, 59-232135, 62-158739, 62-241938 and 62-241941). Also, instead of blending, it was proposed to laminate polyamide layers and polyolefin resin layers to produce multi-layer plastic fuel tanks having improved gas barrier properties (Japanese Patent Laid-Open No. 55-121017 and Japanese Patent Publication No. 60-14695). In this case, since the polyolefin resin layers generally have a relatively poor bonding strength to the polyamide layers, they are laminated via modified plastic layers such as those made of polyolefin resins modified with unsaturated carboxylic acids or their derivatives. Such multi-layer moldings for fuel tanks are generally produced by forming parisons by a usual extrusion molding method and then subjecting the parisons to blow molding (Japanese Patent Laid-Open Nos. 58-220738, 62-110526).
Japanese Patent Publication No. 60-34461 discloses a method of producing a multi-layer, blow-molded container constituted by a laminate of a polyolefin resin layer, a polyamide or saponified ethylene-vinyl acetate copolymer layer and an intermediate layer interposed therebetween, the intermediate layer being made of a modified ethylene copolymer produced by grafting 0.01-1 weight % of maleic anhydride to a copolymer of ethylene and an .alpha.-olefin having 3 or more carbon atoms and showing a crystallinity of 2-30%.
However, the polyamide layers of the above multi-layer moldings are usually made of polyamide 6, and from the viewpoint of moldability of the polyamide layer, the temperature of the high-density polyethylene layer (main layer) should be higher than that of a single-layer plastic fuel tank made of high-density polyethylene only. As a result, there arises a problem of drawdown of a parison.
Apart from the above, since large amounts of flashes, defective moldings, etc. are generated in the course of blow molding, it was proposed to recover those flashes, defective moldings, etc. and reuse them by mixing them with polyolefin resins (Japanese Patent Laid-Open Nos. 54-113678 and 55-91634).
Japanese Patent Laid-Open Nos. 55-91634 discloses a multi-layer molding constituted by a laminate of (a) a polyolefin resin layer, (b) a polyamide layer obtained by melt-blending (i) a modified polymer produced by grafting maleic anhydride to an ethylene/.alpha.-olefin copolymer having a crystallinity of 1-35% and a melt index of 0.01-50 g/10 minutes (maleic anhydride content: 0.05-1 weight %) and (ii) polyamide, and (c) a modified polyolefin resin layer made of a modified polymer produced by grafting maleic anhydride to an ethylene/.alpha.-olefin copolymer having a crystallinity of 2-30% and a melt index of 0.01-50 g/10 minutes (maleic anhydride content: 0.01-1 weight %), which is interposed between the polyolefin resin layer and the polyamide layer for bonding them, the multi-layer molding suffering from only little deterioration of mechanical strength even though flashes, defective moldings, etc. of the multi-layer moldings are reused as part of starting materials.
However, thermoplastic resin compositions having satisfactory bonding strength and impact resistance cannot necessarily be obtained simply by mixing the flashes, defective moldings, etc. of multi-layer moldings with polyolefin resins. This is due to a low compatibility between polyolefin resins and polyamides. Since the impact resistance is a particularly important property for the fuel tanks, its decrease greatly reduces the values of the fuel tanks. Although a high impact resistance can be maintained if flashes, defective moldings, etc. are not added as regenerated products, the cost reduction of materials for the fuel tanks cannot be achieved.
In addition, when flashes, defective moldings, etc. generated in the production of multi-layer plastic fuel tanks are blended with a virgin polyolefin resin, the resin temperature should be set at 230.degree.-250.degree. C. to improve the dispersibility of polyamide. In this case, there is remarkable decrease in drawdown resistance of the parison.